3 files changed, 429 insertions, 0 deletions
diff --git a/18/src/expression.lalrpop b/18/src/expression.lalrpop
new file mode 100644
index 0000000..49356f6
--- /dev/null
+++ b/18/src/expression.lalrpop
@@ -0,0 +1,34 @@
+use crate::types::{Pair, Value};
+
+grammar;
+
+pub Pair: Pair = {
+  LEFT_BRACKET <left:Value> COMMA <right:Value> RIGHT_BRACKET => {
+    Pair {
+      left: left,
+      right: right,
+    }
+  },
+};
+
+Value: Value = {
+  NUMBER => {
+    Value::Literal(<>.parse::<i64>().unwrap())
+  },
+  Pair => {
+    Value::Pair(Box::new(<>))
+  },
+};
+
+match {
+  r"\p{Zs}+" => { },
+
+  r"[0-9]+" => NUMBER,
+
+  "," => COMMA,
+
+  "[" => LEFT_BRACKET,
+
+  "]" => RIGHT_BRACKET,
+}
+
diff --git a/18/src/main.rs b/18/src/main.rs
new file mode 100644
index 0000000..df21bd0
--- /dev/null
+++ b/18/src/main.rs
@@ -0,0 +1,363 @@
+use advent_lib::prelude::*;
+
+use crate::types::{Pair, Value};
+#[macro_use] extern crate lalrpop_util;
+
+pub mod types;
+lalrpop_mod!(pub expression);
+
+
+#[derive(Debug, Clone)]
+enum Step {
+  Left,
+  Right,
+}
+
+
+fn main() -> Result<()> {
+  let mut args = std::env::args();
+  if args.len() != 2 {
+    eprintln!("Usage: advent input");
+  }
+  let _ = args.next();
+  let filename = args.next().unwrap();
+
+  let input = advent_lib::read_lines_file(&filename)?;
+
+  let parser = expression::PairParser::new();
+
+  let mut result = None;
+  for line in &input {
+    let expression = Box::new(parser.parse(line)?);
+
+    match result {
+      None => {
+        result = Some(expression);
+      }
+      Some(previous_result) => {
+        let mut new_expression = add(previous_result, expression);
+        reduce(&mut new_expression);
+        result = Some(new_expression);
+      }
+    }
+  }
+
+  let output = magnitude(&result.unwrap());
+  println!("{}", output);
+
+  let mut expressions = Vec::new();
+  for line in &input {
+    expressions.push(Box::new(parser.parse(line)?));
+  }
+
+  let mut max_sum = None;
+  for i in 0 .. expressions.len() - 2 {
+    for j in i + 1 .. expressions.len() - 1 {
+      let left = expressions[i].clone();
+      let right = expressions[j].clone();
+      let mut sum = add(left, right);
+      reduce(&mut sum);
+      let sum = magnitude(&sum);
+      match max_sum {
+        None => {
+          max_sum = Some(sum);
+        }
+        Some(old_sum) => {
+          if sum > old_sum {
+            max_sum = Some(sum);
+          }
+        }
+      }
+    }
+  }
+
+  println!("{}", max_sum.unwrap());
+
+  Ok(())
+}
+
+
+fn add(left: Box<Pair>, right: Box<Pair>) -> Box<Pair> {
+  Box::new(Pair {
+    left: Value::Pair(left),
+    right: Value::Pair(right),
+  })
+}
+
+
+fn reduce(expression: &mut Box<Pair>) {
+  loop {
+    match find_four_x_nested(&expression, Vec::new(), 0) {
+      None => { }
+      Some((path, left_addend, right_addend)) => {
+        match subtree_to_left(&path) {
+          None => { }
+          Some(left_path) => {
+            let subtree = get_subtree(expression, &left_path);
+            add_to_rightmost_value(subtree, left_addend);
+          }
+        }
+
+        match subtree_to_right(&path) {
+          None => { }
+          Some(right_path) => {
+            let subtree = get_subtree(expression, &right_path);
+            add_to_leftmost_value(subtree, right_addend);
+          }
+        }
+
+        replace_with_zero(expression, &path);
+
+        continue;
+      }
+    }
+
+    if perform_split(expression) {
+      continue;
+    }
+
+    break;
+  }
+}
+
+
+fn find_four_x_nested(expression: &Pair, path_so_far: Vec<Step>, depth: usize)
+  -> Option<(Vec<Step>, i64, i64)>
+{
+  if depth >= 4 {
+    match (&expression.left, &expression.right) {
+      (Value::Literal(left), Value::Literal(right)) => {
+        return Some((path_so_far, *left, *right))
+      }
+      //_ => { panic!("you don't say"); }
+      _ => { return None; }
+    }
+  }
+
+  match &expression.left {
+    Value::Literal(_) => { }
+    Value::Pair(left) => {
+      let mut path = path_so_far.clone();
+      path.push(Step::Left);
+      let result = find_four_x_nested(left.as_ref(), path, depth + 1);
+      if result.is_some() {
+        return result;
+      }
+    }
+  }
+
+  match &expression.right {
+    Value::Literal(_) => { }
+    Value::Pair(right) => {
+      let mut path = path_so_far.clone();
+      path.push(Step::Right);
+      let result = find_four_x_nested(right.as_ref(), path, depth + 1);
+      if result.is_some() {
+        return result;
+      }
+    }
+  }
+
+  None
+}
+
+
+fn subtree_to_left(path: &Vec<Step>) -> Option<Vec<Step>> {
+  let mut result = path.clone();
+
+  loop {
+    match result[result.len() - 1] {
+      Step::Left => {
+        result.pop();
+
+        if result.len() == 0 {
+          return None;
+        }
+      }
+      Step::Right => {
+        result.pop();
+        result.push(Step::Left);
+        return Some(result);
+      }
+    }
+  }
+}
+
+
+fn subtree_to_right(path: &Vec<Step>) -> Option<Vec<Step>> {
+  let mut result = path.clone();
+
+  loop {
+    match result[result.len() - 1] {
+      Step::Left => {
+        result.pop();
+        result.push(Step::Right);
+        return Some(result);
+      }
+      Step::Right => {
+        result.pop();
+
+        if result.len() == 0 {
+          return None;
+        }
+      }
+    }
+  }
+}
+
+
+fn get_subtree<'a>(pair: &'a mut Box<Pair>, path: &Vec<Step>) -> &'a mut Value {
+  let mut here: &mut Box<Pair> = pair;
+
+  for (i, step) in path.iter().enumerate() {
+    if i == path.len() - 1 {
+      match step {
+        Step::Left => {
+          return &mut here.left;
+        }
+        Step::Right => {
+          return &mut here.right;
+        }
+      }
+    } else {
+      match step {
+        Step::Left => {
+          match &mut here.left {
+            Value::Pair(left) => {
+              here = left;
+            }
+            _ => { panic!("left is not a pair"); }
+          }
+        }
+        Step::Right => {
+          match &mut here.right {
+            Value::Pair(right) => {
+              here = right;
+            }
+            _ => { panic!("right is not a pair"); }
+          }
+        }
+      }
+    }
+  }
+
+  panic!("empty path");
+}
+
+
+fn add_to_rightmost_value(subtree: &mut Value, addend: i64) {
+  match subtree {
+    Value::Literal(value) => {
+      *value += addend;
+    }
+    Value::Pair(pair) => {
+      add_to_rightmost_value(&mut pair.right, addend);
+    }
+  }
+}
+
+
+fn add_to_leftmost_value(subtree: &mut Value, addend: i64) {
+  match subtree {
+    Value::Literal(value) => {
+      *value += addend;
+    }
+    Value::Pair(pair) => {
+      add_to_leftmost_value(&mut pair.left, addend);
+    }
+  }
+}
+
+
+fn replace_with_zero(pair: &mut Box<Pair>, path: &Vec<Step>) {
+  let mut here: &mut Box<Pair> = pair;
+
+  for (i, step) in path.iter().enumerate() {
+    if i == path.len() - 1 {
+      match step {
+        Step::Left => {
+          here.left = Value::Literal(0);
+        }
+        Step::Right => {
+          here.right = Value::Literal(0);
+        }
+      }
+    } else {
+      match step {
+        Step::Left => {
+          match &mut here.left {
+            Value::Pair(left) => {
+              here = left;
+            }
+            _ => { panic!("left is not a pair"); }
+          }
+        }
+        Step::Right => {
+          match &mut here.right {
+            Value::Pair(right) => {
+              here = right;
+            }
+            _ => { panic!("right is not a pair"); }
+          }
+        }
+      }
+    }
+  }
+}
+
+
+fn perform_split(expression: &mut Box<Pair>) -> bool {
+  match &mut expression.left {
+    Value::Literal(n) => {
+      if *n >= 10 {
+        expression.left = Value::Pair(Box::new(Pair {
+          left: Value::Literal((*n as f64 / 2.0).floor() as i64),
+          right: Value::Literal((*n as f64 / 2.0).ceil() as i64),
+        }));
+        return true;
+      }
+    }
+    Value::Pair(left) => {
+      if perform_split(left) {
+        return true;
+      }
+    }
+  }
+
+  match &mut expression.right {
+    Value::Literal(n) => {
+      if *n >= 10 {
+        expression.right = Value::Pair(Box::new(Pair {
+          left: Value::Literal((*n as f64 / 2.0).floor() as i64),
+          right: Value::Literal((*n as f64 / 2.0).ceil() as i64),
+        }));
+        return true;
+      }
+    }
+    Value::Pair(right) => {
+      if perform_split(right) {
+        return true;
+      }
+    }
+  }
+
+  false
+}
+
+
+fn magnitude(expression: &Box<Pair>) -> i64 {
+  let mut result = 0;
+
+  result += 3 * match &expression.left {
+    Value::Literal(n) => *n,
+    Value::Pair(left) => magnitude(left),
+  };
+
+  result += 2 * match &expression.right {
+    Value::Literal(n) => *n,
+    Value::Pair(right) => magnitude(right),
+  };
+
+  result
+}
+
diff --git a/18/src/types.rs b/18/src/types.rs
new file mode 100644
index 0000000..6800fdc
--- /dev/null
+++ b/18/src/types.rs
@@ -0,0 +1,32 @@
+use std::fmt;
+
+
+#[derive(Clone)]
+pub enum Value {
+  Literal(i64),
+  Pair(Box<Pair>),
+}
+
+#[derive(Clone)]
+pub struct Pair {
+  pub left: Value,
+  pub right: Value,
+}
+
+
+impl fmt::Debug for Value {
+  fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+    match self {
+      Value::Literal(n) => { write!(formatter, "{}", n) }
+      Value::Pair(pair) => { write!(formatter, "{:?}", pair) }
+    }
+  }
+}
+
+
+impl fmt::Debug for Pair {
+  fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+    write!(formatter, "[{:?},{:?}]", self.left, self.right)
+  }
+}
+